The present invention relates to screw compressors. More particularly, the present invention relates to screw compressors employed in refrigeration chillers. With still more particularity, the present invention relates to the prevention of oil backflow out of a screw compressor in a refrigeration chiller and the loss of oil to the system evaporator as a result thereof.
Screw compressors are compressors in which two or more screw rotors are disposed in an intermeshing relationship in a working chamber. The counter-rotation of the screw rotors draws gas into the working chamber at a first, relatively low pressure, causes the compression of such gas within the working chamber and causes the discharge of such gas at a higher, so-called discharge pressure therefrom.
In many screw compressor applications, including application in refrigeration chillers, oil may be injected directly into the compressor's working chamber for cooling and sealing purposes. Additionally, oil is used to lubricate the compressor bearings. Oil used for bearing lubrication in refrigeration chillers is typically vented/directed to a location within the compressor where refrigerant gas at a relatively low pressure is found. Such oil will, therefore, eventually make its way into the compressor's working chamber and become entrained in the refrigerant gas that flows through it. Such oil, together with any oil that was injected directly into the compressor's working chamber, is then carried out of the compressor entrained in the flow stream of gas discharged from the compressor.
Because the flow stream of refrigerant gas issuing from a screw compressor in a refrigerant chiller contains a relatively large amount of oil and because such oil needs to be returned to the compressor for the various purposes mentioned above, an oil separator is typically located in or immediately downstream of the compressor for the purpose of disentraining the oil from the discharge gas flow stream and gathering it for return to the compressor. In many chiller systems, it is the discharge pressure found in the oil separator that is used to drive the separated oil from the oil separator back to the compressor.
While the oil separates used in such systems are very highly efficient, typically disentraining 99% or more of the oil from the refrigerant gas flowing therethrough prior to the exit of the gas for delivering to the system condenser, it will be appreciated that over time the compressor's oil supply can come to be depleted. Any oil that makes its way past the system oil separator is typically carried into and through the system condenser and winds up in the system evaporator pooled on or in the liquid refrigerant that will be found at the bottom thereof. Provisions are typically made for regularly returning this relatively small amount of oil from the system evaporator back to the system compressor, such oil migration, once again, being typical in refrigeration chillers of all types and typically involving only a relatively very small amount of oil as a percentage of the chiller's oil supply.
Because there is a direct flow path from the chiller's evaporator to the chiller's compressor component through which suction gas is drawn into the compressor, the possibility does exist, under some conditions, for oil located within the compressor to flow out of the compressor, in a direction back toward and even into the evaporator. Such conditions are somewhat unique to and are exacerbated in refrigeration chillers that employ screw compressors due to the amount of oil which is used for various purposes within such compressors and due to the fact that the system evaporator is located below and generally in an open flow relationship with the suction area of the compressor in such systems. Oil flow directly into the system evaporator from the compressor, while atypical, can sometimes be in quantities greater than it is the capacity of the oil return apparatus associated with the evaporator to cope with and can result in chiller shutdown for lack of oil in sufficient quantity in the proper location to ensure that the compressor is continuously and adequately supplied with oil while in operation.
Exemplary of previous arrangements by which such oil is caught and trapped for return to the compressor in a refrigeration chiller after backflowing thereoutof are those found in U.S. Pat. Nos. 5,086,621 and 5,396,784. The '621 patent addresses the oil backflow problem by positioning a tray within the evaporator beneath the piping through which suction gas is drawn from the evaporator to the compressor. That tray catches and accumulates any backflowing oil. Such oil is then returned on a continuing basis to the system compressor by use of the eductor apparatus.
The '784 patent likewise teaches the positioning of a tray beneath the evaporator outlet in a refrigeration chiller to catch and return backflowing oil. In the '784 patent, however, when the level of oil in the tray becomes sufficiently high, gas flow from the evaporator to the compressor comes to be restricted with the result that gas flow velocity is caused to increase. The increased flow velocity of the gas flowing out of the evaporator to the compressor causes the entrainment of oil located in the tray in the gas stream flowing out of the evaporator back to the compressor.
As will be appreciated, both such arrangements require the fabrication and installation of parts/components which are assembled into the system evaporator to address the oil backflow problem. Such parts/components, their fabrication and installation come at significant expense and their operation comes at some expense in terms of the overall power consumed by the chiller system.
The need continues to exist for an arrangement by which to prevent the backflow of oil from a screw compressor to the evaporator in a refrigeration chiller system which does not add significantly to the expense of the compressor or chiller system and which does not penalize chiller efficiency.